High pressure decorative laminates are conventionally produced by stacking and curing under heat and pressure a plurality of layers of paper impregnated with various synthetic thermosetting resins. In normal practice the assembly from the bottom up, consists of a plurality, e.g. three to eight, core sheets made from phenolic resin impregnated kraft paper, above which lies a pattern or print sheet impregnated with melamine resin; on top of the print sheet is provided an overlay sheet which, in the laminate, is almost transparent and provides protection for the pattern sheet.
The core sheets are conventionally made from kraft paper of about 90-125 pound ream weight. Prior to stacking, the kraft paper is impregnated with a water-alcohol solution of phenol-formaldehyde resole, dried and partially cured in a hot air oven, and finally cut into sheets. The print sheet is a high quality, 50-125 ream weight, pigment filled, alpha cellulose paper that has been impregnated with a water-alcohol solution of melamine-formaldehyde resin, dried and partially cured, and finally cut into sheets. The print sheet, prior to impregnation with the resin, usually has been printed with a decorative design, or with a photogravure reproduction of natural materials, such as wood, marble, leather, etc.
The overlay sheet is almost invariably used when the print or pattern sheet has a surface printing in order to protect the printing from abrasive wear. The overlay sheet is a high quality alpha cellulose paper of about 20-30 pounds ream weight that is also impregnated with melamine-formaldehyde resin in a manner similar to that used for the print sheet, except that a greater amount of resin per unit weight of paper is used. The individual sheets are stacked in the manner indicated above and, if six sheets of impregnated core paper are used, there results a finished laminate having a thickness of about 50 mils, it being understood that a different number of sheets can be used to provide thicker or thinner laminates.
The stack of sheets as described above is placed between polished steel plates and subjected to about 230.degree.-340.degree. F. (e.g. 300.degree. F.) at 800-1600 p.s.i. (e.g. 1000 p.s.i.) for a time sufficient to consolidate the laminate and cure the resins (e.g. about twenty-five minutes). This causes the resin in the paper sheets to flow, cure and consolidate the sheets into a unitary laminated mass referred to in the art as a decorative high-pressure laminate. In actual practice, two laminated stacks are pressed back to back, separated by a coated release sheet that allows the two laminates to be peeled apart after separation. Also, a large proportion of the stacks are laminated with an aluminum foil-kraft paper composite sheet inserted between the overlay and the metal plate, with the aluminum facing the overlay, in order to obtain a laminate having a lower gloss and a slightly textured surface which is desirable for some products.
At the completion of the laminating operation, the backs of the laminates are sanded to permit gluing to particle board, plywood or other substrates. The glued, laminate surfaced panel is then fabricated into furniture, kitchen counter tops, table tops, store fixtures and other end-use applications widely accepted for the combination of appearance, durability and economy.
A number of variations of the above-described general process are known, particularly those operations designed to obtain special effects in appearance and texture. Also other curing cycles are possible and, in fact, sometimes other resin systems are used as well.
Besides decorative high-pressure laminates referred to above, there are also a number of low-pressure products which have been developed in recent years, including low-pressure laminates using either saturated polyester resins, or melamine-formaldehyde resin. One of the fastest growing materials competing with high-pressure laminates in recent years is a product referred to as low-pressure melamine board which is normally pressed in a short cycle at 175-225 p.s.i. at 325.degree.-350.degree. F. These low-pressure products have the advantage of being normally less expensive, but they cannot be given the title of "high pressure laminates" because in order to be entitled to that designation, a product must meet a variety of rigid standards promulgated by the National Electric Manufacturers Association, NEMA LD3-1975 which includes standards relating to abrasive wear, stain resistance, heat resistance, impact resistance, dimensional stability, etc. While various other decorative printed, surfacing materials, such as some of the low-pressure laminates, have certain of the desirable characteristics, no products other than high-pressure laminates currently available have all of these properties.
One of these properties in particular which is very important is abrasion resistance. A high-pressure decorative laminate must have sufficient abrasion resistance to permit use in high exposure areas such as dinette surface tops, check-out counters, etc. The standard NEMA test for abrasion resistance is NEMA test LD-3.01. In this test a laminate sample is clamped on a rotating disc, over which ride two weighted rubber wheels, faced with calibrated sand-paper strips. As the laminate surface is rotated under the wheels, the abrasive action of the sand-paper cuts through the surface of the laminate and gradually through the overlay until the printed pattern is exposed and destroyed. The NEMA standard for Class I laminate requires that the laminate, after four hundred rotation cycles, has no more than 50% of its pattern destroyed. The 50% end point is estimated by averaging the number of cycles at which the pattern shows initial wear, and the number of cycles at which the pattern is completely destroyed.
If a high-pressure decorative laminate is prepared in a conventional manner, with a normal 35-40% resin content in the print or pattern sheet, but without an overlay sheet, the abrasion resistance will be only about 50-75 cycles. If specially formulated melamine resins are used in the pattern sheet with a resin content of 50-55%, abrasion resistance of up to about 150-200 cycles are on occasion obtainable without an overlay sheet, but in this latter case the laminates have a tendency to develop surface craze and, furthermore, they are quite difficult to prepare due to the difficulty of impregnating the print sheet in a uniform manner; additionally, they do not meet the 400 cycle minimum required by the NEMA standard.
Nevertheless, it is desirable to produce a laminate without an overlay sheet which is capable of attaining the performance characteristics of a laminate using an overlay, and, in particular, one that provides a 400 cycle abrasion resistance. Furthermore, it is desirable to provide a laminate which, in addition to having the 400 cycle abrasion resistance, has an initial wear point at least equal to the initial wear point of a conventional high-pressure laminate having overlay, typically 175-200 cycles. This is desirable because in actual use the laminate appearance becomes unsatisfactory not when 50% of the pattern is destroyed, but when a much lower percentage is destroyed. It is well known from many years of field experience that conventional laminates with overlay, which have 175-200 cycle initial wear point, when used in hard use areas, will have a satisfactory appearance, at least as long as the normal replacement cycle, it being understood that replacement of most laminates in commercial uses is made for style reasons rather than because of pattern wear. Therefore, a laminate without overlay should meet these same criteria, namely it should have both a NEMA abrasion resistance of at least 400 cycles and an initial wear point in the same test of at least 175-200 cycles, even though the latter requirement is not part of the NEMA standard.
It is desirable to be able to provide these characteristics, but without using an overlay, for several reasons:
1. Overlay adds substantial raw material costs to the manufacture of laminates, both the cost of the overlay paper itself, the cost of the resin used to impregnate the overlay paper and the in-process and handling losses of these materials.
2. The overlay, by imposing an intermediate layer of substantial thickness between the print sheet and the eyes of the viewer, detracts significantly from the desired visual clarity of the pattern. The cellulose fibers used to make overlay paper have a refractive index close to that of cured melamine-formaldehyde resin. The fibers are therefore almost invisible in the cured laminate, and permit the printed pattern to be seen with very little attenuation. However, modern printing techniques are making available very accurate reproductions of natural materials, particularly various wood veneer species. As these printed reproductions approach in appearance the natural veneer, even small amounts of haze or blur introduced by the overlay paper are disturbing visually and destroy much of the realism desired by the user.
3. Furthermore, the overlay contributes to the rejection rate of the laminate products produced. The impregnated, dry overlay sheet tends to attract small dirt particles because it develops static electricity charges during drying. This dirt is hard to detect and remove before laminating, and results in spoiled laminate sheets that cannot be reprocessed. In addition, the impregnated dried overlay is brittle and hard to handle without breakage. Broken pieces are accidentally trapped on the surface of the overlay and also result in visually defective sheets.
Additionally, overlay containing laminate, particularly those having a relatively high surface gloss, have a tendency to become dull very quickly when subjected even to only moderate abrasive wear. This is understandably unacceptable where glossy laminates are desired.
The problem of providing improved abrasion resistance has been a long standing problem in the field. Many solutions to the problem have been suggested and, in fact, some of these have reached commercial development. Nevertheless, it has not heretofore been possible to provide a laminate, not having an overlay sheet, but having a NEMA abrasion resistance of least 400 cycles and an initial wear point in the same test of at least 175-200 cycles.
It is well known that small, hard mineral particles dispersed in overlay paper, or in resin mixtures to coat the impregnated pattern sheet, can enhance the abrasion resistance of high-pressure laminates (see, for example, the patents to Michl, U.S. Pat. No. 3,135,643; Fuerst U.S. Pat. No. 3,373,071 and Fuerst U.S. Pat. No. 3,373,070). Techniques such as these do not eliminate the overlay, but either enhance its abrasion resistance, or provide an alternate form of overlay and associated resin.
For example in the Barna U.S. Pat. No. 3,123,515, the overlay sheet is impregnated with a finely divided frit, the impregnated sheet containing between 20 and 60% by weight of resin and frit in which the proportion of frit is between about 35 and 60% of the total solids added. The overlay is used in the normal manner by placing it over the print or pattern sheet.
In the Fuerst U.S. Pat. No. 3,373,070, a process is disclosed whereby silica is incorporated into the overlay structure during the manufacture of the overlay paper itself, thereby providing a uniform distribution of the silica throughout the overlay sheet. This patent includes a discussion near the bottom of column 1 of the disadvantages of the Barna type procedure of impregnating the overlay, Fuerst being of the opinion that a silica rich resinous coating on the top of the overlay is undesirable.
The Michl U.S. Pat. No. 3,135,642 in essence shows the casting of, or the in situ manufacture of, an overlay sheet over the print sheet. The coating includes silica, finely divided cellulose flock, carboxy methyl cellulose and melamine resin solids. The weight of the dry coating is said to be 0.022 to 0.033 pounds per square foot of print sheet on the dry basis. This weight is equivalent to 66-99 pounds per ream, corresponding almost exactly to the weight of a conventional impregnated overlay paper, and has a thickness of about 2.5 mils (see Table D of Michl). At best the Michl procedure provides only a minor raw material cost advantage compared with the use of conventional overlay, and does not solve the problem of impaired visual effects due to haze or blur.
The Fuerst U.S. Pat. No. 3,373,071 is very similar to the Michl patent, except that the overlay cast in situ over the print sheet contains micro crystalline cellulose. This coating is said to be applied, on a dry weight basis, of 0.022 to 0.33 pounds per square foot, again giving a thick coating which weighs at least 66 pounds per ream, the same weight as the conventional impregnated overlay paper.
One interesting technique which was unsuccessfully tried is that disclosed in the Lane et al U.S. Pat. No. 3,798,111 in which there is disclosed the use of small mineral particles, preferably alumina, which are incorporated within and near the upper layer of the base paper during its manufacture. Thus, the abrasive-resistant particles are incorporated in the paper during the paper-making process as in Fuerst '070, but, more analogously to Barna, they are incorporated after the base layer of paper has been formed and is still in a wet state supported on the forming wire. After its manufacture, this paper is subsequently printed, impregnated and then used in the laminating operation as the print sheet without the necessity of using an overlay. In this process, the printing occurs above or on top of the hard mineral particles and, consequently, high-pressure laminates produced using a print sheet made in accordance with the Lane patent, and without an overlay, have unacceptably low initial wear, even though they do have a NEMA abrasion resistance of at least 400 cycles. In tests, it has been shown that laminates made with the print paper of Lane et al, without overlay, had initial wear values of under 100 cycles, some as low as 35 cycles. Furthermore in a rubbing test to determine initial wear, such laminates began to show pattern destruction after only 3,000 rub cycles, far less than necessary.
Other prior art patents of some interest with regard to the background of the present invention are the patents to Fuerst U.S. Pat. No. 3,445,327; Gibbons U.S. Pat. No. 3,928,706 which suggests the use of a cast in situ overlay used together with a conventional overlay, and Merriam U.S. Pat. No. 3,661,673. Of somewhat less interest are the Battista U.S. Pat. Nos. 3,259,537 and 3,157,518; Ando et al U.S. Pat. No. 3,716,440; Power et al U.S. Pat. No. 3,946,137 and Boenig U.S. Pat. No. 3,318,760.
Even after the considerable activity in the field in order to solve the problems indicated above, these problems have not been solved until the present time.